Q&A: Installing Asphalt Shingles on a Low-Slope Roof

Q.What is the minimum slope
allowed for asphalt shingles? I have heard that asphalt
shingles can be applied on a roof with a pitch as low
as 2/12, as long as a rubberized asphalt eaves membrane
is installed under the shingles.

A.Corresponding Editor
Paul Fisette replies: The CABO One and Two
Family Dwelling Code permits the installation
of asphalt shingles on roofs with a slope as low as
2/12 if the shingles are double-coverage,
self-sealing versions.

Shingles that are not self-sealing must be
hand-sealed with asphalt roofing cement. This
low-slope application requires a double layer of
#15 felt underlayment, cemented together, for a
distance extending from the eaves up to a point 24
inches inside the interior wall line of the
building. For the rest of the roof slope, the felt
underlayment must be installed with a 19-inch top
lap and a 12-inch side lap, providing double
coverage of the felt layer.

Asphalt shingle manufacturers will warrant
shingles installed on roof slopes as low as 2/12
when these special installation procedures are
followed. Self-sticking rubberized asphalt products
like Grace Ice & Water Shield are acceptable
but expensive substitutes for #15 felt.

Having said this, I would never install shingles
on a 2/12 roof, especially in an area where snow
falls. In fact, I have had numerous problems in
northern climates with asphalt shingles on 3/12
roofs. As a result of this experience, I would
limit the application of asphalt roof shingles to
roofs that are 4/12 and steeper. The National
Roofing Contractors Association recommends that
asphalt shingles be installed only on roofs with
slopes that are 3/12 and greater.

I believe that if you have to depend on felt
paper, asphalt cement, or Ice & Water Shield
to block water after it gets past the primary roof
covering, you have the wrong roof system. Shingles
get brittle with time, and at the reduced slope of
a 2/12, a slight curl at the end of the shingle
line shunts water backwards under the leading edge
of the overlapping shingle. When you have snow or
ice sitting on the roof and melting, you
don’t have a prayer at keeping the water
out. Felt paper and Ice & Water Shield are
emergency backups, not a plan for everyday
protection.